This invention relates to Stirling engine displacer suspension systems, and more particularly to an oil filled system for suspending a free displacer in a free piston Stirling engine.
This invention is related to application Ser. No. 172,373 for "Diaphragm Displacer Stirling Engine Powered Alternator-Compressor," filed on July 25, 1980, by Folsom, et.al., and application Ser. No. 270,974 for "oil-Backed Stirling Engine Displacer Diaphragm" filed by me concurrently herewith, and application Ser. No. 270,892 for "Sealed Oil-Backed Displacer Suspension Diaphragm" filed by Nicholas Vitale concurrently herewith, the disclosures of which are all incorporated herein by reference. The engine of the '373 application is a free piston Stirling engine which utilizes a diaphragm to suspend the displacer in the working space and uses the pressure wave in the working space to maintain the displacer oscillation. Although the machines disclosed in these applications constitute significant progress in the art, there are some areas in which modifications would improve overall performance.
In the machine of the '373 application, the displacer diaphragm is subjected to stress induced by the pressure swing of the working gas in the working space which is on the order of 10% to 20% of the charge pressure in the working space which can be on the order of 40-80 bar which, acting over the full face of the diaphragm, can introduce considerable stress in the diaphragm. This complicates the deformation pattern of the diaphragm and reduces its working life. This pressure induced stress does not contribute to the operation of the machine. The only stress that contributes desirable design function is displacement induced stress, that is, the spring effect contributed by the diaphragm when it is displaced from its central position. This necessary and desirable stress in the diaphragm is compounded and multiplied in unpredictable ways and with deleterious results by pressure induced stresses in the diaphragm which greatly complicates the diaphragm design and reduces diaphragm reliability and repeatability. Moreover, the effect changes with changing pressure and therefore an additional degree of difficulty is introduced if a power control system based on mean pressure variation is used.
A second desirable area of improvement is control of the power input into the displacer itself. Power input into the displacer is related to the displacement ratio (.DELTA.V/V) of the difference .DELTA.V between the volumetric displacements of the displacer in the expansion space and the compression space, to the volumetric displacement V of the expansion space. The power required to maintain the oscillation of the displacer in the working space, that is to overcome the friction of the moving displacer and windage losses of the working gas flowing through the heat exchangers, normally requires a (.DELTA.V/V) value of approximately 0.1. However, the deflection pattern of a diaphragm normally produces a value of (.DELTA.V/V) in an engine of this variety of approximately 0.3. This provides excessive energy to the displacer so that it slams back and forth between its stops unless some means is provided to extract the excess energy, or some technique is provided for limiting the (.DELTA.V/V) ratio, and hence the input energy to the displacer, to a value more suited to the engine operation.
Along these lines, it is possible by artful design of a displacer to enable it to operate with the characteristic desired, that it, with a small (.DELTA.V/V). Nevertheless, there is no assurance that the diaphragm will actually operate in this manner when mounted in the engine and operated at various pressures and other operating parameters, even though it is capable of doing so. Therefore, it is necessary to impose a form of restraint on the deformation pattern of the diaphragm in its operation so that it will conform to the desired configuration.
Another possible improvement would be to enhance the diaphragm life and reliability by reducing the stress which the diaphragm must carry. The diaphragm acts as a spring element to store energy when the displacer is displaced from its center position and to utilize the stored energy by returning the displacer from its stroke extremes toward the center position. The stress level in the diaphragm approaches its maximum at the displacer stroke extremes. At these extremes, it would be desirable to have a supplementary spring member assume a portion of the load so that the diaphragm need not carry the entire load.
Yet another improvement that I believe would be desirable would be to eliminate the mechanical connection between the displacer and the diaphragm, so that the diaphragm experiences only distributed fluid pressure forces and not concentrated mechanical forces. This design change should improve the diaphragm life, simplify its design, and reduce its cost.